Tokkai Hei 5-57148 published by the Japanese Patent Office in 1993 regarding the purification of hydrocarbon (HC) in the exhaust of an internal combustion engine, discloses an arrangement wherein plural catalytic converters comprising hydrocarbon adsorbents (HC adsorbents) and catalysts which oxidize hydrocarbons are disposed in series in an exhaust passage of an engine. The HC adsorbents absorb hydrocarbons when the temperature is low, and discharge the hydrocarbons when the temperature is high. The catalyst is activated when the temperature rises, and then oxidizes the hydrocarbons.
When the catalyst activation temperature is higher than the temperature at which HC adsorbent starts to discharge hydrocarbons, a specific region will be created where hydrocarbons released from the HC adsorbent are not oxidized.
As the converter temperature generally decreases the further away the converter is from the engine, the HC adsorbent of the downstream converters will still not have reached the HC discharge start temperature even if the upstream converters reaches the aforesaid specific temperature region.
Therefore, hydrocarbons flowing out of the upstream converter are adsorbed by the HC adsorbent of the downstream converters.
In this case, during the interval from when the temperature of the converter situated furthest downstream reaches the aforesaid specific temperature region to when it rises above the specific temperature region, hydrocarbons will be discharged into the atmosphere.
However, the discharge amount is less than the discharge amount of hydrocarbons discharged by the upstream converter discharges, and the amount of hydrocarbons which is finally discharged is less the larger the number of converters connected in series.